It is known in the art relating to internal combustion engines that aluminum bore engines can provide benefits such as improved fuel economy due to the lighter weight of aluminum engines and better engine performance and reliability due to reduced bore distortion and improved heat transfer through aluminum cylinder bores. Aluminum bore engines also allow tighter clearance between piston and cylinder bore, leading to a reduction in engine noise and oil consumption. When aluminum materials are used for both the piston and the cylinder bore, however, scuffing resistance between pistons and cylinder bores becomes a significant issue. Scuffing resistance is of further importance in aluminum bore engines because of the tighter clearances allowed between aluminum pistons and bores.
Scuffing is an adhesive wear process that occurs when two parts slide against each other in a lubricant starved condition. Scuffing between pistons and cylinder bores is problematic as it may lead to decreased engine performance and may cause engine failure. A current aluminum piston skirt, machined with a single diamond point in a continuous cut, typically has a wavy surface with peaks and valleys. Roughness total (Rt) is a characteristic of piston skirts defined as the distance from the highest peak to the lowest valley measured over a specified length of the piston skirt. A current aluminum piston skirt may have a roughness total of between 13 and 19 microns. Further, a current piston skirt may typically have a distance between peaks of 0.32-0.79 mm (320-790 microns). Conventionally, to improve scuffing resistance between pistons and cylinder bores, piston skirts are coated with a coating material that provides scuffing resistance. The conventional piston skirt coating process involves coating the piston skirt without altering the conventional roughness total of the piston skirt or the distance between peaks. In fact, it has been commonly believed that reducing the surface roughness of the piston skirt (lowering the roughness total and the distance between peaks) is ineffective in improving scuffing resistance, and may even hinder scuffing resistance.
U.S. Pat. No. 6,684,844 B1 discloses the results of earlier work for the assignee of the present invention pertaining to improved scuffing resistance of an improved coated aluminum piston operated in a cast-iron cylinder bore. Development of an optimum aluminum cylinder bore treatment and material for use with the improved piston to obtain improved wear and scuffing resistance was desired.